I can remove it and all the changes that I've done with this tag, if it's necessary. Please see my response on the forum-

I have posted the topic "Extraction of the ISO3166-1 capital cities" on the forum, so you're invited to see it and discuss it.

Hello,
they aren't documented. How can I do it?
The main administrative entities in OSM are referenced by ISO3166 standard (boundary=administrative). I want to make a world political map from OSM data and every capital city of an ISO3166 territory should have that attribute. I can't extract that information from existing tags. Where could I explain this?
Thanks in advance